1. Field of the Invention
The present invention relates to an image processing apparatus for performing conversion, correction or the like on image data.
2. Description of the Prior Art
A color image processing apparatus is known which uses a masking method for printing a color image of good quality. In this masking method, masking equations of higher order (e.g., Clapper equations) are frequently used for performing high quality color correction.
However, among such equations of higher order only equations of second or third order have been used heretofore and a satisfactory color image cannot always be reproduced. In addition to this, as it is difficult to set the coefficients for such equations it can only be performed by an experienced technician. When there is a change in the input optical system (lens system) for reading an original image, color of the illumination system, color temperature of the background of the transfer sheet, or the like, the coefficients must then be updated.
Various methods have been proposed for producing halftone images with a digital printer or the like.
The dither and density pattern methods are examples of such various methods.
These methods are widely adopted for the following reasons:
(1) A halftone image can be reproduced by a binary display device. PA1 (2) Hardware construction of the system is simple. PA1 (3) An image of considerably satisfactory quality can be obtained. More specifically, as shown in FIGS. 1-A and 1-B, an input pixel (input pixel information) 58 corresponds to each element of a threshold matrix 55. Whether the pixel is to be printed black or white is determined by comparing to determine if the input pixel is larger than the threshold value. The obtained data is supplied to a display screen 56. PA1 1. 4.times.4 for resolution unit PA1 2. 8.times.8 for gray level unit
FIG. 1-A shows the dither method wherein an input pixel 58 corresponds to an element of the threshold matrix 55. FIG. 1-B shows the density pattern method wherein one input pixel 58 corresponds to all the elements of the threshold matrix 55. In other words, in the density pattern method, a single input pixel 58 is indicated by a plurality of cells at the display screen 56.
In this manner, the dither method and density pattern method differ from each other only in that one input pixel corresponds to one element of the threshold matrix in the former while one input pixel corresponds to all the elements of the threshold matrix in the latter. Thus, these two methods are basically the same. A method intermediate to these two methods is also conceivable. According to such a method, an input pixel corresponds to a plurality of elements (e.g., a 2.times.2=4 elements in FIG. 1-B) among all the elements of the threshold matrix.
Since these methods are basically the same, the dither method, the density pattern method and the intermediate method will inclusively be called the dither method hereinafter. In such a dither method, there are various methods of preparing a threshold matrix. Not much research has been conducted on methods of producing with ease images of high quality. However, a method of producing a halftone image with improved quality without degrading the resolution using the threshold matrix having the format as shown in FIG. 2-A is known. This threshold matrix has a format of:
FIG. 2-B shows the initial states of printing of recording dots when this threshold matrix is used. When input image data has a uniform density and an image is produced using this threshold matrix, a matrix pattern in units of 8 dots is formed when L=1 and a matrix pattern (which may also be referred to as a matrix arrangement) is formed inclined at 45.degree. when L=2 in FIG. 2-B. However, when L=3, a uniform matrix pattern is not formed, but becomes nonuniform as shown in FIG. 3. The resultant matrix pattern is also non-uniform when L=5, 7 or the like.
When such a nonuniform (irregular) pattern (arrangement) is to be developed by electrophotography, density irregularity tends to be caused and the gray levels are disturbed when the recording dot pitch spatially changes. When such an image is printed with an ink jet printer or the like, non-uniformity of the arrangement of the recording dots becomes apparent.
Various conventional apparatuses have been proposed to perform color correction of color image data. For example, an apparatus is known which uses as an address a tricolor input digital signal from a scanner and performs color conversion and color correction in accordance with the table index method.
However, in this apparatus, the capacity of the table memory becomes extremely large, and the ratio of the cost of the memory to the total cost of the apparatus becomes prohibitively high.
FIG. 4 shows an example of a conventional apparatus of this type. Tricolor input digital data 1a, 1b and 1c from a color reader are supplied to a color conversion memory 53 as address data. Output data 7a, 7b and 7c from the memory 53 become the data after tricolor conversion. When it is assumed that the input digital signals each consist of m bits, each color signal has a combination of states of 2.sup.m. Therefore, the color space which can be expressed by tricolor synthesis is 2.sup.3m. It is also assumed that output data for each color consists of m bits.
When the above respects are considered, the color conversion memory 53 needs a capacity of 2.sup.3m bits for address, and a capacity of 3m bits for output data. As a result, the total memory capacity N must be N=2.sup.3m .times.3m bits.
When m is assumed to be 6, N is calculated to be 4,718,592 bits, that is, about 590 kbytes.
When m is assumed to be 8, N is calculated to be 402,653,184 bits, that is, about 50 Mbytes. Thus, the memory capacity of the color conversion memory 53 becomes extremely large, resulting in an increase in the apparatus cost.